Field of the Invention
The instant invention relates generally to an improved deep-bed filter used for filtering water, sewage or other aqueous liquids, and specifically to an improved apparatus and method for liquid filtration utilizing a polymeric filtration media layer for enhanced contaminant filtration with minimum pressure drop through the filter.
The concept of liquid filtration utilizing multimedia deep bed filters is well known in the art. Prior art filters often employ layers of granular media from the coarsest to the finest for efficient filter operation. While most suspended contaminants can be removed from a liquid by filtering through a deep enough bed of fine granular filter media, a high pressure drop across the filter and the necessity of frequent filter backwashing to remove surface contaminant buildup render such filters impractical for use in municipal and industrial filtration applications where tremendous volume and contaminant concentrations exist.
Additionally, deep bed filters often have difficulty handling peak or emergency loads without over-design of the filter or the addition of an auxiliary filter to handle the extra contaminant load. Simply adding layers of filtration media to existing deep bed filters to solve the aforementioned problems is ineffective. Furthermore, backwashing many prior art filters having media of varying particle sizes but identical specific gravities results in a reversal of the media grading order, i.e., from small to large.
This reverse gradation problem has been solved to some degree by using media materials having differing specific gravities. However, even when using materials of such different specific gravities as anthracite and sand, if the granules of coal are large enough they will stratify at lower levels within the filter bed. These aforementioned dual media filters are generally used to handle increased turbidity loads and will provide longer periods of filter operation between backwashing. However, when turbidity gets very high and coagulants must be used these filters are still subject to surface binding, thereby requiring frequent backwashing.
The primary problem with known in the art dual media deep bed filters is that large coagulated particles and floc that are larger than the voids in the top layers of media are strained at the surface instead of passing into the depth of the media. This buildup of surface contaminants on the filter causes pressure buildup on the filter surface, thereby restricting flow of high turbidity liquids and preventing effective use of adequate chemical flocculating agents. Furthermore, during filter backwashing media classification takes place and the coal fines settle at the surface of the filter, thereby closing off any voids and further restricting fluid flow through the filter.